The insulating nature of anodic oxide films on the so-called valve metals has been known since the mid 1800's. These films were investigated systematically by Guntherschulze during the first 3 decades of the 20th century and the results are described in the work, Electrolytkondensatoren, by Guntherschulze and Betz (published by Verlag Von M. Krayn, Berlin, 1937). A more thorough and up to date work describing the subject is Anodic Oxide Films, by L. Young (published by Academic Press, London and New York, 1961).
When the more ideal examples of the valve metals, such as tantalum, are anodized in appropriate electrolytes, the amount of oxide growth is proportional to the current passed, with very small losses due to electronic leakage current through the anodic oxide. At a fixed electrolyte solution temperature, the thickness of the anodic oxide films on valve metals is proportional to the anodizing voltage. In 1955, Torrisi demonstrated that the anodic oxide film thickness at fixed voltage is proportional to the Kelvin (absolute) temperature of the anodizing solution and that for a given film thickness:(T1)(V1)=(T2)(V2),  (1)where T=Kelvin Temperature and V=Applied Voltage.
This relationship was found to hold over the range of 5–500 volts and 0–200° C. [A. F. Torrisi; Journal of the Electrochemical Society. 102, 176, (1955).]
In 1970, Dreiner and Tripp published a study of the anodizing kinetics of tantalum in aqueous alkali metal anodizing solutions in a pressure bomb over the temperature range of 0–250° C. [R. J. Dreiner and T. Tripp, Journal of the Electrochemical Society. 117, 858, (1970).] These researchers found that the oxide thickness (amount of anodic oxide produced) is proportional to the current passed up to about 170° C. Above 170° C. the anodic oxide was found to exhibit a significant electronic leakage current (i.e., current passing through the film without further, or at least efficient, film growth), probably due to oxygen migration from the oxide film to the tantalum substrate, resulting in a semi-conducting rather than insulating oxide film. The electronic leakage current results from a degradation of the insulating nature of the film In 1997, Melody, Kinard, and Lessner discovered the conditions under which non-thickness-limited (N-T-L) anodic oxide film growth takes place. In N-T-L film growth, the oxide grows thicker at 50+% current efficiency at fixed voltage while producing a non-porous, insulating film at temperatures in excess of about 150° C., N-T-L film growth, which is a major departure from traditional anodizing kinetics, is described in the following publications:                Electrochemical and Solid State Letters, Vol. 1, No. 3, pages 126–129, Title: “The Non-Thickness-Limited Growth of Anodic Oxide Films on Valve Metals”, by Brian Melody, Tony Kinard, and Philip Lessner (1998)        Journal of the Electrochemical Society, Vol. 148, No. 9, pages B337–342, Title: “Non-Thickness-Limited Growth of Anodic Oxide Films on Tantalum”, by Y. -M. Li and L. Young (2001)        
U.S. Pat. Nos. 5,837,121 , 5,935,408 , 6,149,793 , 6,235,181, and 6,267,861 assigned to the assignee of this invention, are relevant and are hereby incorporated by reference.
The insulating action of anodic oxide films is to act as highly resistive coatings but they are permanently damaged by the passing of current through them at voltages above the so-called withstanding voltage of the films. (The withstanding voltage, or voltage which the anodic oxide is capable of withstanding without permanent damage/high current flow, is usually equal to or less than the anodizing voltage.) Except under very unusual circumstances, such as those found in N-T-L anodizing, the application of voltages below the withstanding voltage of an anodic oxide film results in the passing of only very low currents through the insulating film.
For some applications, it is useful to be able to pass an electric current thorough the insulating anodic oxide film covering a valve metal anode body with the anode body biased with a positive voltage and without destroying the insulating nature of the oxide nor growing significant amounts of new anodic oxide. One useful effect which may be realized by the passage of current from a power supply through the anodic oxide coating on anodized valve metal anode bodies with the anode bodies biased positive is the electrolytic production of intrinsically conductive polymer films such as polypyrrole, polythiophene or polyaniline and derivatives thereof on the surfaces of the anodic oxide coatings. Such conductive polymer coatings covering anodic oxide films on valve metal anode bodies are useful for the production of electrolytic capacitors, resistors, switches, sensors, etc. Alternately, if the anodic oxide coating on the anodized valve metal body is biased positive and the oxide is rendered conductive per the methods of the present invention, then metal salts may be added to the electrolyte system and the anode bodies can be biased negative resulting in the passage of current through the anodic oxide coating on anodized valve metal anode bodies that can be utilized for the electrolytic plating of metals such as silver, gold, copper or zinc onto the surfaces of the anodic oxide coatings using a soluble salt. While the metal cations are attracted to the negatively biased conductive oxide, the halogens or other anions are repelled from the oxide surfaces. In short, the passage of the non-destructive, non-oxide forming current of the present through the oxide coatings on anodized valve metal anode bodies may be useful for various different electrochemical processes known to those in the art.
It is an objective of this invention to render the oxide of a valve metal conductive. It is a second objective of this invention to render the oxide of a valve metal reversibly conductive without denigrating from its insulative characteristics.
It is a third objective of this invention to be able to produce novel and more effective cathodes for capacitors.
It is yet another objective of this invention to produce sensors for specific chemical compounds and temperature sensitive switches.
These and other objectives can be met by placing an anodized valve metal in a solution which is an anhydrous, polar aprotic liquid system at a reduced temperature and using the oxidized valve metal as one electrode.
The paper on non-thickness-limited anodizing by Melody, Kinard, and Lessner, referenced above, establishes the role of hydroxyl groups (present in anodic oxide films) in the stabilization of these films, at least over the temperature range examined by the authors. The reversible nature of N-T-L anodizing, with its dependence on electrolyte solution water content, strongly suggests that the hydroxyl content of anodic oxide films is not constant, but may be reduced by the application of a high electric field (valve metal biased positive) under conditions which do not allow hydroxyl groups to enter the film as quickly as they are eliminated from the film (presumably through dissociation of protons from the oxygen atoms making up hydroxyl groups with rapid removal of the protons from the oxide due to the high electric field).